The Monisteres, who have described themselves as founding members of the “Who Dat Nation,” claimed they trademarked and nurtured the phrase after they enlisted several Saints players to chant it on a recording of “When the Saints Go Marching In,” sung by Aaron Neville.

In the days leading up to the 2010 Super Bowl, the NFL sent out a batch of cease-and-desist letters demanding T-shirt makers stop selling “Who Dat?” merchandise. But the NFL backed down after state officials challenged the league’s trademark infringement claims.

Mark Andews, a lawyer for Storyville Apparel, said the settlement surprised and disappointed his client and other T-shirt sellers. The NFL had promised to help defend them against Who Dat? Inc’s claims and “do the heavy lifting” in preparing for a trial, Andrews added.

“It pulls the rug out from under us,” he said.

The T-shirt sellers’ attorneys say Who Dat? Inc. lost whatever trademark rights it may have had years before the Saints won the Super Bowl. They argue “Who Dat?” has been a slogan in the public domain for decades and can’t be trademarked.